The present invention relates to a high density ammonium nitrate-fuel oil (xe2x80x9cANFOxe2x80x9d) blasting composition comprising an organic liquid fuel and porous prilled ammonium nitrate of high bulk density, small particle size and good oil absorption capacity. By xe2x80x9chigh densityxe2x80x9d in the previous sentence is meant a poured bulk density of from about 0.90 to about 1.05 g/cc.
ANFO blasting compositions are the most widely-used explosives in the world today. They are relatively simple to manufacture and are comprised of basically two commercially available components: porous prilled ammonium nitrate (xe2x80x9cANxe2x80x9d) and organic liquid fuel such fuel oil or diesel fuel. The AN and fuel oil are mixed generally in a stoichiometric weight ratio of 94% AN and 6% fuel oil, and in fact, porous prilled AN (xe2x80x9cPPANxe2x80x9d) is conveniently capable of absorbing about 6% fuel oil. Although ANFO is relatively insensitive and thus safe to handle, it becomes a powerful blasting composition once it is properly initiated. Nevertheless, it has certain disadvantages in blasting applications.
One disadvantage is that ANFO is water-soluble and thus cannot be used reliably in water-containing boreholes unless it is packaged or otherwise segregated from water. Due to the size and porosity of the prills, ANFO has a relatively low bulk density of about 0.85 g/cc. This makes packaged ANFO difficult to use in water-filled boreholes because of the relative bulk densities and the tendency of the packaged ANFO to float in water. The low bulk density of ANFO also can be disadvantageous where explosives having higher energy densities are desired for particular blasting applications.
Efforts have been made to increase the density of ANFO-type blasting compositions. AN or ANFO prills have been combined with a liquid matrix such as an emulsion phase to form a xe2x80x9cheavy ANFO.xe2x80x9d The liquid matrix complicates, however, the manufacturing of the blasting composition since a stable emulsion first must be manufactured. Other higher density materials or components have been added to ANFO in an effort to increase its density. For example, aluminum particles have been added to increase both bulk and energy density. A different approach involved the pneumatic ejection and packing of ANFO into a borehole, although this resulted in a packing density of only about 0.94 g/cc.
Another approach has been to use high density AN prills (xe2x80x9cHDANxe2x80x9d), rather than low density, porous AN prills (PPAN). High density AN prills (HDAN) are commonly referred to as agricultural grade and are used primarily as fertilizer. They are manufactured by a different process than that used to make low density, porous AN prills and consequently the high density prills are considerably less porous and have insufficient oil absorption capacity for use effectively in an ANFO blasting composition.
Efforts also have been made to improve the oil absorption capacity of high density prills. U.S. Pat. No. 4,736,683 describes these efforts and itself describes a further effort comprising the addition of a high molecular weight polymer to improve fuel retention of the fuel on high density AN particles. Related U.S. Pat. Nos. 5,486,246 and 5,527,498 disclose a porous AN matrix, and a method for making the matrix, of higher density than porous AN prills and having a xe2x80x9chighxe2x80x9d oil absorption capacity. The method involves adding an internal additive and an external coating to the high density prills.
The high bulk density, porous prilled AN (xe2x80x9cHBDPPANxe2x80x9d) of the present invention has comparable, if not superior, oil absorption capacity to standard PPAN and does not require the addition of polymers or of special additives or coatings in order to improve polymers or of special additives or coatings in order to improve oil retention properties. By. xe2x80x9chigh bulk densityxe2x80x9d is meant a poured bulk density of from about 0.9 to about 1.0 g/cc.
During the manufacturing and handling of PPAN, a small percentage of xe2x80x9cfinesxe2x80x9d are generated. These fines are small diameter particles that have a higher bulk density than PPAN and generally contain a relatively higher weight percentage of the anti-caking additives that are present in PPAN. These fines tend to destabilize an emulsion phase and thus normally are separated from the PPAN and sold for less value as AN fertilizer. It has been found in the present invention, however, that these fines actually function as HBDPPAN in ANFO. Their small particle size contributes to their high bulk density. Their porosity and thus oil absorption capacity results from their method of manufacture, which method of course is the same as the method for manufacturing PPAN, and from the increased surface area of the smaller prills. Thus another advantage of the present invention is the more beneficial use of fines.
The invention comprises a higher density ANFO blasting composition comprising an organic liquid fuel and HBDPPAN having a particle size of less than about 1.2 mm and an oil absorption capacity of greater than 5%. Preferably, the HBDPPAN comes from fines generated during the manufacturing or handling of PPAN.